Scanning antenna feed



P. FOLDES SCANNING ANTENNA FEED Jan. 2l, 1969 Sheet \3 Filed sept. 1o,1964 INVENTOR Pfff/f Fowfs BY v v Jan. l21, 1969 l Filed Sept. l0, 1964P. FOLDES SCANNING ANTENNA FEED sheet 4 of 4 INVENTQR.

Pfff: Hawes United States Patent O 3,423,756 SCANNING ANTENNA FEED PeterFeldes, Montreal, Quebec, (Ianada, assignor to Radio Corporation ofAmerica, a corporation of Delaware Filed Sept. 10, 1964, Ser. No.395,401

U.S. Cl. 343-775 11 Claims Int. Ci. Htllq 19/14 ABSTRACT F THEDISCLGSURE An electronically controlled conical scanning antenna feed isprovided by an oversized waveguide having four tuned cavities mountedabout the waveguide and coupled to it. The signal of the frequency towhich these cavities are tuned is split into higher order modes thusresulting in the movement of the radiation phase center from the centerof the antenna aperture. By tuning the four cavities in sequence to thefrequency of this signal, it is conically scanned. Signals at otherfrequencies if sufficiently separated from the frequency to which thecavities are tuned continue to propagate through the waveguide withoutany disturbance within the waveguide.

My invention relates to antennas and particularly to an improved feedsystem for deflecting the antenna pattern. In a preferred embodiment ofthe invention the pattern is deflected so as to conically scan withoutthe utilization of mechanical movement of any large component, or evenwithout any mechanical movement.

The invention will be described with particular reference to an antennadesigned for use as a ground station antenna for a communicationssatellite system. Such an antenna should transmit and receivecommunication signals lying within a wide frequency band, and it shouldalso transmit a signal having a radiation pattern or patterns having acharacteristic such that when directed to a target such as a satellitean error signal is provided which may be utilized to make the antennaautomatically track the target. Thus, the communication signal patternis always directed toward the satellite.

The desired antenna tracking may be obtained, for eX- ample, byemploying a monopulse antenna or by employing a conical scanningantenna. The latter has certain advantages; for example, it requiresonly one receiver.

The present invention, as applied to a communications satellite system,is an improved conical scanning antenna. Stated more specilically, it isan improved antenna feed which, in the example to be described, feeds aCassegrainian antenna. The conical scanning is obtained withoutmechanically moving either the antenna reflectors or the feed. Also, ifdesired, only -a narrow frequency band signal in the vicinity of afrequency fB is conically scanned; the communication signals at adifferent frequency fc which are transmitted or received by the antennanot being affected by the conical scan. Thus, in this case, there isprovided an optimum gain characteristic for the communication signals.

An object of the invention is to provide an improved feed system for anantenna.

A further object of the invention is to provide an improved conicalscanning radiating system.

A further object of the invention is to provide an improved means fordeflecting an antenna radiation pattern.

A still further object of the invention is to provide an improved meansfor deliecting a radiation pattern without the mechanical movement ofany large component.

A still further object of the invention is to provide an improved feedor radiating system that wil-l radiate energy over a wide band offrequencies within a certain radiation 3,423,756 Patented Jan. 21, 1969pattern and that will deflect only the radiation pattern of a narrowband of frequencies within said wide band.

In practicing one embodiment of the invention, the antenna feedcomprises an oversized waveguide to which the signals are fed from awaveguide dimensioned to propagate the signals in the TEN mode, forexample. The signals propagate through the oversized waveguide in thissame mode only so long as no asymmetry or discontinuity is introduced inthe oversized waveguide.

A signal at any preselected frequency passing through the antenna feedmay be made to radiate in a radiation pattern (beam) that conicallyscans by utilizing four bandstop filter cavities that are mounted aroundthe oversized waveguide, each cavity mounted asymmetrically with respectto a symmetry plane of the waveguide, and each cavity coupled to thewaveguide when the cavity is tuned to resonate at the said preselectedfrequency. Such coupling introduces an asymmetrical discontinuity in theoversized waveguide so that the mode TEN, in the instant example, issplit into the additional modes 'IEZO and TEM and TMH. This results indeflection of the beam radiated or propagated from the oversized guide.By proper-ly tuning the four cavities in time succession to resonate atthe selected frequency, the signal at this frequency has its radiationpattern deected so that it scans on a circular cone surface.

The present invention is particularly useful for ground stations of acommunications satellite system because communication signals at widelyspaced frequencies can be passed through the antenna feed without beingdeected, while a signal at a different frequency may at the same time bepassed through the feed and its pattern made to conically scan so as toprovide error signals which may be employed to make the antennaautomatically track the communications satellite. The communicationsignals being transmitted to or received from the satellite by theantenna are not affected by the conical scanning.

The invention will be described in detail with reference to theaccompanying drawing, in which:

FIG. l is a side view of an antenna feed or radiating system constructedin accordance with one embodiment of the invention;

FIG. 2 is a view taken on the -line II-II of FIG. l looking in thedirection of the arrows;

FIG. 3 is a cross-sectional view of the oversized waveguide and thebandstop filter cavities shown in FIG. 1, this view being taken on theline III-III of FIG. 4 looking in the direction of the arrows;

FIG. 4 is a view taken on the line IV-IV of FIG. 3 looking in thedirection of the arrows;

FIG. 5 is a view illustrating a Cassegrainian antenna that is fed by anantenna feed embodying the present invention;

FIG. 6 is a schematic diagram illustrating one way of so varying theresonant frequency of the bandstop filter cavities of FIG. l as toprovide conical scanning;

FIG. 7 is a view showing a `portion of the feed system of FIG. ltogether with an illustration of radiation patterns that are referred toin explaining the invention;

FIG. 8 is a pair of graphs representing radiation patterns which arereferred to in explaining the invention;

FIG. 9 yis a schematic diagram illustrating another way of so varyingthe resonant frequency of the bandstop filter cavities of FIG. l as toprovide conical scanning;

FIG. l() is a cross-sectional view of the oversized waveguide withbandstop filter cavities mounted thereon in accordance with anotherembodiment of the invention, this view being taken on the line X-X ofFIG. 11 looking in the direction of the arrows;

FIG. 11 is a top view of the waveguide and cavities shown in FIG. 10;

FIG. 12 is a block diagram of a transmitter-receiver in which thepresent invention is incorporated; and

FIG. 13 is a top view of the oversized waveguide with bandstop filtercavities mounted thereon in accordance with another embodiment of theinvention.

In the several figures, like parts are indicated by similar referencecharacters.

FIGS. 1 and 2 illustrate an embodiment of the invention designed toprovide conical scanning. More specically, it is designed to passsignals within a wide frequency band such as desired at the groundstation of a communication satellite system, and is designed toconically scan the radiation pattern of a narrow band of energy withinsaid wide frequency band. For example, the antenna feed of FIG. 1 willpass any communication signal from 5000 megacycles per second to 7500megacycles per second; and within this frequency band a signal in thevicinity of 6000 megacycles per second may have its radiation patternconically scanned.

The following description will be with reference to transmission orradiation from the feed system. However, on the basis of reciprocitytheorem this is also a description of the operation when the feed systemis receiving signal, as from a satellite. In the case of reception ofsignals, the signal that is conically scanned (the above-mentionedsignal in the vicinity of 6000 mc., for example) may be the beaconsignal from a satellite which is acted upon by conical scanning in thefeed system to produce an error signal which may `be utilized to makethe antenna track the satellite.

In the embodiment shown in FIGS. l and 2, the signal is :passed througha transmission line which preferably is either a square or a rectangularwaveguide. In the present example it is a square waveguide. The largestside dimension of the waveguide 10 is greater than one-half 7\ and isless than A where x is the Wavelength of the lowest practical frequencyto be passed. The signal is propagated down the line 10 in the TEm mode,and is passed in that -mode through an impedance matching or transitionsection 11 to a square oversized waveguide 12. The length of thetransition section 11 preferably is from three to five times x so thatreflection from the transition remains negligible. The waveguide 12preferably feeds into a pyramidal square horn 13 which, in the preferredembodiment, radi- -ates the signal to the hyperboloid of a Cassegrainianantenna (see FIG. 5). The horn 13 may also be described as an oversizedwaveguide which is flared. If desired, the oversized waveguide 12 andthe horn 13 mayl be circular. They should have two orthogonal symmetryplanes. With reference to FIG. 5, it is not drawn to scale. Actualtypical values are: Diameter of paraboloid=300 wavelengths; diameter ofhyperboloid=30 wavelengths; aperture size of horn =3 wavelengths.

In order to provide conical scanning, four bandstop filter cavities 14a,14b, 14C, and 14d are mounted on the four sides of the oversizedwaveguide. If preferred, these cavities may `be mounted at the throatcross-section of the horn or on the horn itself. These cavities andtheir mounting are shown in more detail in FIGS. 3 and 4 which show thatthere is a coupling hole for coupling each cavity to the oversizedwaveguide. This coupling is effective, however, only for signal atsubstantially the frequency at which the cavities are resonant. Thus,signals at frequencies not close to the frequency at Which the cavitiesare resonant are propagated down the oversized waveguide in the TEN modeand are radiated in a radiation pattern which is not deflected.

The action of the bandstop filter cavities will now be described withparticular reference to FIGS. 3 and 4. The oversized waveguide 12 isdimensioned so as to support the four waveguide modes TEN, TEZU, and TEMand TMm. These modes can be propagated for either vertical polarizationor horizontal polarization or both. The inside width a (and also heightsince the waveguide is square) is greater than )t and less than 1.5Kwhere A is the longest operational wavelength. By longest operationalwave length is meant the wavelength of the signal of the lowestpractical frequency to be propagated by the feed system. In the examplebeing described, this frequency is 5000 megacycles per second. Thelongest practical wavelength is usually limited by losses in thewaveguide. By the use of a ridge-loaded waveguide this wavelength can bemade greater.

The four bandstop filter cavities and their coupling to the oversizedwaveguide are alike. Referring to the cavity 14a as shown in FIGS. 3 and4, it is on the top of the waveguide 12 and is coupled to the waveguide12 through a circular hole 16a. The cavity 14a and its coupling hole aremounted off-center with respect to the center axis of the top of thewaveguide 12 (with respect to the vertical symmetry plane) so that thecavity 14a is coupled asymmetrically to the wave guide 12.

If the bandstop filter cavity 14a is tuned to a frequency in thevicinity of a frequency B which is not close to the operationalfrequency fc, the cavity has practically no effect on the propagation offrequency fc signal in the waveguide 12. As an example, if the cavity istuned to a -frequency of 6005 mc. and the operational frequency fc is a6010 mc. carrier wave carrying voice channels, the 6010 mc. carrier`wave is substantially `unaffected by the cavity. In this case theincoming 6010 mc. signal being propagated in the TEN mode travelsthrough the waveguide 10 (FIG. l), the matching section 11, thewaveguide 12 and the horn 13 without any disturbance, and the horn 13radiates a symmetrical pencil beam 17 (radiation pattern) as illustratedin FIGS. 7 and 8 in solid line. Thus, any conical scanning of theradiation pattern of a signal at a frequency is the vicinity of fB (f3being 6000 mc. in this example) that is produced by the action of thecavities 14a, 141), 14o and 14d does not deflect or otherwise affect thesignal at frequency fc (6010 mc. in this example). The minimumseparation between fB and fC which provides unaffected operation for fcdepends upon the geometry of the bandstop cavities.

Now consider the case where conical scan is desired so that an errorsignal is obtained which may be employed to make the antennaautomatically track a target such as a satellite. The four cavities 14a,etc., are tunable through a frequency range from fm to BZ where thecenter frequency is fB, which in this example is 6000 mc. In thisexample it will be assumed that fB1i=6005 rnc. and that fB2=5995 mc. Asignal of frequency fm, for example, is propagated down the antenna feed(FIG. l) in the TEM, mode to the oversized waveguide 12. Now the signalat frequency fm as it passes through the Waveguide 12 is affected lbyone or more of the cavities 14a, etc., since they are tuned successivelyto the frequency fm and are, therefore, successively coupled towaveguide 12 at this frequency. Since the coupling is asymmetrical withrespect to waveguide 12, the incoming TEN mode splits into theabove-mentioned four modes, namely, TEN, TE20, and TEM and TMm. Thesefour modes are represented by graphs in FIG. 3. The T1310 and T1220graphs represent the voltage read from a probe that is movedhorizontally through the waveguide. The TE11+TM11 graph is for the sumof these two modes and represents the voltage read from a probe that ismoved vertically through the waveguide. The presence of these modesresults in an asymmetrical field distribution in the antenna feed fromvthe cavity coupling to the output aperture of the horn 13.

Referring for the moment only to the effect of the one cavity 14a whenit is resonant at frequency fm and the signal frequency is fm, theasymmetry can be characterized by an amplitude and phase asymmetry, andfurthermore by a movement of the radiation phase center from the centerof the aperture. This shift in the position of the phase center isillustrated in FIG. 7 where the radiation pattern 17a in broken linesrepresents the pattern of the fm frequency signal that is beingdeflected by the cavity 14a. Also, it will be noted that the radiationpattern 17a is tilted otf axis.

The frequency JBl at which the cavity is resonant, and at which theradiation pattern of a signal is tilted, can be adjusted by selectingthe geometrical dimensions b, c and h of the cavity (see FIGS. 3 and 4),and by selecting the amount t that a tuning pin 18a extends into thecavity.

The amount of angular tilt 0 of the radiation pattern can be controlledby the diameter d of the coupling hole 16a andby the distance x that thecenter of the coupling hole is away from the center line of the top ofthe oversized waveguide 12.

If the input signal to waveguide 12 is dual or circularly polarized thenone arrangement that is possible to obtain coupling to both the TEM, andTEM modes is one that uses four cavities with the one cavity on eachside being rotated as shown in FIGURE 1. Still referring only to theelfect of a single cavity, the tilt of the radiation pattern occurs in aplane indicated in FIG. 3 as P0. The angle that the plane P0 makes witha horizontal plane passing through the center of the waveguide 12 isidentified as the angle This angle of the plane in which the tilt occurscan be controlled by the angle a shown in FIG. 4. It will be seen thatthe angle et is the angle at which the wide side of the cavity is tiltedwith respect to a plane drawn perpendicular to the longitudinal axis ofthe oversized waveguide 12.

In the example illustrated, the angle a is adjusted so that the angle ofthe p-lane in which the tilt occurs is 45 degrees. In this example theangle a is 12.5 degrees. Therefore the radiation pattern or beam istilted in the same plane regardless of whether the exciting wave in thewaveguide 12 is the TEN mode or the TEO, mode. In this case the antennafeed may =be used for either linear or circular polarizations. If linearpolarization is used, the desired conical scanning at the frequency fmmay be obtained regardless of the particular value of the angle or and,therefore, of the angle [3.

In order that conical scanning may be provided, rather than deflectionin one plane, four cavities are mounted around the oversized waveguide12, one cavity on each side of the cavity as shown in FIGS. 1 to 4. Thefour cavities indicated at 14a, 14h, 14C and 14d are mountedsymmetrically around the waveguide 12, but with each cavity mountedasymmetrically with respect to the symmetry plane. Each cavity is aduplicate of the other, and each cavity is mounted on and coupled to thewaveguide the same as described in connection With the cavity 14a.

Assume a signal at frequency fm is propagated down the antenna feed andis to be conically scanned. As shown in FIGS. 3 and 9, in order toprovide conical scanning, movable tuning pins 18a, 18b, 18C and 18d areprovided for the cavities 14a, 141i, 14e and 14d, respectively. Each pinis mounted so that it may be slid in and out of its cavity. The pins areslid in and out sinusoidally with a 90 degree or one quarter time perioddelay in the motion of the pin in consecutive cavities. There are FAcomplete periods per second, FA preferably being an audio frequency. Onesuitable drive to provide such motion for the tuning pins is illustratedin FIG. 9. As shown in FIG. 9, each tuning pin is driven by a crank, andthe cranks are phased so that when pin 18a is fully in the cavity (andthe cavity resonant at frequency fBl), the pin 18h is one-half way in,the pin 18e is completely out (180 degrees out of phase with pin 18a),and the pin 18d is one-half way in. At this position of the tuning pinsthe cavity 14a is producing the entire deflection of the signal fBIradiation pattern, the cavity 14a` is producing no deection since it isnot resonant to the signal at frequency fBl, and the cavities 14b and14d are each producing less than maximum deection but in equal amountsand in opposite directions so that their deflections cancel.

In the example being described, each cavity is resonant at frequency fBZ(5995 mc. in this example) when its tuning pin is completely out.Therefore a signal at frequency im propagated down the antenna feed willbe yconically scanned as previously described by driving the tuning pinsas shown in FIG. 9.

A signal at the center frequency fB, the frequency lat which a cavity isresonant with its tuning pin half way in, will not be deflected bydriving the tuning pins. However, a signal at any frequency between fBand fm or between fB and fm may be conically scanned by driving thetuning pins, although the cone will not be a circular one. In practicethis deviation of the cone from the circular has no importance becauseit only makes the horizontal and vertical error signals slightlydifferent, and it causes a slight reduction in the error voltage becausethe audio filter in the detector circuit for this voltage removes thehigher harmonics 0f the error signal which result from a noncircularcone scanning. At frequencies fm and fBg the cone of scanning ispractically circular.

The communication frequency band of this antenna system when used forcommunication ideally is cSBz-i 0f cfBi-i-fi where f1 is the 3 dbbandwidth of a cavity and is With fc so located, a signal at either fBlor fm, or at a frequency between fBl and fBZ, may be conically scannedwithout deecting (conically scanning) the communication signal.

By tolerating a small amount of conical scanning loss, the communicationfrequency band fc can be in the frequency band of scanning; for example,fc might be somewhere between fB1 and fm. In general, this is not apreferred arrangement, but it is a useable one for communication.

FIG. 6 illustrates an arrangement for obtaining conical scanning withouteven a minor mechanical motion. Instead of tuning pins, each cavity isprovided with a ferrite rod and a magnetizing or control coil forchanging the permeability of the ferrite rod. In this case the cavitiesshould be of a non-magnetic material such as brass. Each ferrite rod maybe held in place by a sheet of insulating material, such as Teon,frictionally supported in the cavity.

The magnetizing coils have a sine wave current owing through them whichis supplied from a source 21. The current in the magnetizing coils isdisplaced degrees in consecutive coils by means of the phase Shifters22, 23 and 24. Each cavity is also provided with a bias coil carrying asteady direct current supplied from a D-C source such as a battery. Themagnetic iield of the bias coil causes the ferrite rod to have apermeability intermediate its maximum and minimum permeabilitiesproduced by the sine wave current owing through the magnetizing coil.The conical scanning action is the same as that described in connectionwith FIG. 9. Specifically, taking the instant when maximum positivepolarity iS flowing through the magnetizing coil of cavity 14a, theferrite rod of 14a has maximum permeability (the fields of themagnetizing and bias coils adding) and cavity 14a is tuned to resonanceat frequency B1. At the same time, maximum current of negative polarityis owing through the control coil of 14e and the ferrite rod of cavity14e has minimum permeability (since the sine wave eld cancels the -biasfield) so that cavity 14C is not resonant at frequency fBl. No currentis flowing through the magnetizing or control coils of cavities 14b and14d at this instant so that the ferrite rods in these cavities have thesame permeability, Which is a value intermediate the maximum and minimumpermeabilities. Thus, the deection effects of cavities 14h and 14dcancel out at this instant. All deflection of the radiation pattern atthis instant is caused by the cavity 14a.

In the foregoing examples, the coupling hole or aperture between eachbandstop cavity and the oversized waveguide has been described andillustrated as a circular hole. However, it may have other shapes. Formaximum coupling, the coupling hole should be a slot having a lengthabout equal to a half free space wavelength of the signal to beconically scanned, and the length to width ratio of the slot should beabout ten. Also, the largest coupling is obtained if the coupling holeis about halfway ybetween the symmetry plane and the side wall of thecavity. The coupling is most frequency sensitive when adjusted formaximum coupling as described above. Considerable frequency sensitivitymay be desirable so that the existence of the TE20 and TEM and TMm modescan be restricted to a narrower frequency band.

If the input wave from waveguide is dual or circularly polarized, thearrangement using four cavities at an angle a described in the foregoingexample may be used where the angle a is adjusted so that the cavity hasan equal amount of coupling to both the TEM, and the TEM exciting modes,and the corresponding beam tilt occurs in the diagonal plane of theradiating horn 13.

A second arrangement using eight cavities as shown in FIGS. l0 and 11may be used where the input wave is dual or circularly polarized. Eachcavity is provided with a tuning pin as previously described. In thisarrangement, for a signal to be deflected, the cavities 100, 101, 102and 103 are coupled to the signal being propagated in the" TEU, mode.The cavities 100 and 102 at the top and bottom of the waveguide 12,respectively, have their wide dimension perpendicular to the directionof propagation of the signal. The cavities 101 and 103 at the sides ofthe waveguide 12 have their wide dimensions parallel t0 the direction ofpropagation of the signal.

In the example now being described, the signal is also being propagatedin the TEO1 mode. The cavities 100A, 101A, 102A and 103A are coupled tothe signal in this mode. As illustrated, the cavities 100A and 102A onthe top and bottom of the waveguide 12, respectively, have their widedimension parallel to the direction of propagation of the signal. Thecavities 101A and 103A on the sides of the waveguide 12 have their widedimension perpendicular to the direction of propagation of the signal.

In the case of either dual polarization (illustrated in FIG. l0) orcircular polarization, the actuation of the tuning pins for producingconical scanning is the same as previously described. It may be notedthat when the tuning pin of cavity 100 is extended into its cavity amaximum amount, the tuning pin of its adjacent cavity 100A is alsoextended into its cavity a maximum amount. The same is true with respectto the operation of the tunings pins of the other adjacent cavities.

FIG. l2 illustrates in block diagram a communication or radar systemwhich incorporates an embodiment of the present invention. If employedas a radar system, it may be of the pulse-echo type for tracking atarget and measuring its range.

The system comprises a radio transmitter 51 which feeds signal through adiplexer 52 to the waveguide 10, matching section 11, oversizedwaveguide 12 and multimode horn 13 previously described. The receivedsignal passes through the horn 13, waveguide 12, matching section 11,and waveguide 10, and through the diplexer 52 to a preamplifier 53. Theamplified signal is passed to a mixer 54 where it beats with signal froma local oscillator 56. The resulting IF signal is amplified in amplifier57 and detected by detector 58 to supply the demodulated receivedsignal.

The error signal of frequency ,A resulting from the conical scanning isdetected by an amplitude modulation detector 59 and supplied to abandpass filter 61 that passes the error signal and applies it to phasesensitive detectors 62 and `63. Each of the detectors 62 and `63 issensitive to both the relative phase and the relative amplitudes of thesignals applied to it. Each of these detectors preferably is phaselocked in accordance with well-known practice.

The previously described arrangement for conical scanning employingferrite rods in the bandstop filter cavities is shown in block diagramform. Signal at frequency fA from the oscillator 21 is applied withoutphase shift over a lead 60 to the phase detector 62 where it functionsas a reference signal for the elevation error signal. The output ofdetector `62 is a signal that controls, by a servo system (not shown),the elevation tracking of the antenna being fed `by the multimode horn.Also, signal at frequency fA is applied with degree phase shift over alead 64 to the phase detector 63 where it functions as a referencesignal for the azimuth error signal. The output of detector 63 is asignal that controls the azimuth tracking of the antenna by means of aservo system (not shown).

If the conical scanning is produced Iby means of tuning pins driven asshown in FIG. 9, the reference signals foi the detectors 62 and 63 maybe obtained from signal supplied from a generator coupled to the drivingmotor as illustrated.

As illustrated in FIG. 13, the conical scanning can be restricted to anarrower frequency band by locating more than one cavity along thedirection of propagation of the signal. In the example illustrated, twocavities are located along this direction, these being 14a and 14a1 onthe top of waveguide 12, 14b and 14b1 on one side, and 14d and 14d1 onthe other side. The two cavities on the bottom of waveguide 12 are notshown. The tuning pins for the cavities 14a and 14a1 are indicated at18a and 18a1, respectively. The two cavities in line are tuned to thesame frequency. When a signal is being conically scanned, the tuningpins of the two cavities in line are moved together, the tuning pin 18a1being completely out when the tuning pin 18a is completely out.

If, for some reason, the target tracking is to be achieved in a widefrequency band, then the cavities which are in line along the directionof signal propagation may be tuned to different and equally spacedfrequencies with their frequency spectrums overlapping. There may bethree or more such cavities rather than only the two in line cavitiesshown in FIG. 13.

It may be desirable to design the antenna feed so that the antenna iscapable of tracking any one of several satellites or targets. Suchsatellites may each be radiating a beacon signal at a frequencydifferent from the beacon frequencies of the other satellites. For sucha design the cavities which are in line may be designed to be resonantat widely spaced frequencies located in the overall communicationfrequency band of the antenna.

The narrowness of the frequency band to which the conical scanning isrestricted depends in large part upon how sharply resonant the cavitiesare. A cavity is resonant at a signal frequency if its electrical lengthis equal to one-half the signal wavelength. However, the cavity may bemade more sharply resonant to this frequency if the cavity is given alength that is a multiple of this one-half wavelength.

The bandstop filter cavities have been described as being mounted on theoversized waveguide 12. Instead of being mounted on waveguide 12, theymay be mounted at the throat of the horn 13 or on the multimode horn 13itself. The conical scanning operation is the same whether the bandstopfilter cavities are mounted on the waveguide 12 or on the horn 13. Aspreviously mentioned, the horn 13 is a flared oversized waveguide. Thephrase oversized waveguide in the claims refers to the horn 13 as wellas to the waveguide 12.

It may be noted that the conical scanning system can be optimized forthe tracking signal. For example, assume the antenna feed of FIG. l isoperated with the communication signal having a frequency far enoughremoved from the frequency of the signal being conically scanned so thatthe communication signal is unaffected by the conical scanning. Then theantenna patterns angle of tilt 01, shown in FIG. 8, preferably is madeequal to 0.5 00 where, as shown in FIG. 8, 00 is the half angle betweenthe 3 db points of the pattern. On the other hand,

if the communication signal itself is being conically scanned, then theangle of tilt 01 preferably is given a value between 0.3 and 0.4 00.

There will now be described a way in which -a system, such as thatillustrated in FIG. 12, may be made less sensitive to error caused byamplitude modulation of the received signal due, for example, to fading,target scintillation or target rotation. Instead of rotating the antennabeam at one rate such as 30 cycles per second, it may be rotatedalternately at two different rates such as 30 c.p.s. and 45 c.p.s. Thismay be done as illustrated in FIG. 12 by providing a second oscillator21a and a second ybandpass lter 61a. As an example, the oscillator 21has a frequency of 30 c.p.s. and the oscillator 21a has a frequency of45 c.p.s. Then the filters 6-1 and l61a are designed to pass 30 c.p.s.and 45 c.p.s., respectively. A motor M drives switches as illustrated toconnect alternately the oscillator 21 and filter y61 to the circuit andthe oscillator 21a and filter 61a to the circuit. This switching may bedone, for example, at the rate of once per minute. Thus, an undesiredamplitude modulation of a frequency of approximately 30 c.p.s. due totarget scintillation, for example, will be less likely to prevent theantenna from tracking the target.

Instead of switching as described above, the oscillators 21 and 21a maybe connected to feed simultaneously into the system so the antenna beamwill scan at different rates, i.e., the beam will speed up and slow'down in the course of making several rotations. With this arrangementthe Ibandpass filters 61 and `61a are connected in parallel so thattheir outputs are connected at all times to the two phase sensitivedetectors.

What is claimed is:

1. An antenna -feed for providing angular deliection of signals withinone frequency band while simultaneously passing undeected signals withinother frequency bands, said feed comprising:

a waveguide having two orth'ogonal symmetry planes,

a transmission line connected to said waveguide for propagating to saidwaveguide said signals within said one frequency band and within saidother frequency bands in a dominant mode,

said waveguide being dimensioned so as to support the propagation ofsaid signals in additional modes,

means for only splitting said dominant mode of said signals within saidone frequency band into said additional modes causing deflection of saidsignals within said one frequency band and for simultaneously passingundeflected said signals within said other frequency bands.

2. An antenna feed for providing angular deliection of signals withinone frequency band while simultaneously passing undeflected signalswithin other frequency bands comprising:

an oversized waveguide having tw-o orthogonal symmetry planes,

a transmission line coupled to said waveguide, said line beingdimensioned to propagate signals within said one frequency band andwithin said other frequency bands in at least one of two dominant TEmodes,

said waveguide being dimensioned so that it can propagate said signalsin the additional modes TE20, T1311l and TMm,

and means for introducing asymmetry into said waveguide only for saidsignals within said one frequency band and for simultaneously passingundeliected said signals within said other frequency bands whereby saiddominant mode of said signals within said one frequency band is splitinto said additional TEzo, TEM and TMm modes and said signals withinsaid one frequency band are deflected.

3. An yantenna feed comprising an 'oversized waveguide having twoorthogonal symmetry planes,

a transmission line coupled to said waveguide, said line beingdimensioned to propagate signals in a certain mode,

said waveguide being dimensioned so that it can propagate signals in theadditional modes TEZO, TEM and TMll:

means for introducing asymmetry into said waveguide whereby said certainmode is split into said additional TEm, TEM and TMm modes, said lastnamed means includes a resonant cavity mounted on each side of saidwaveguide, each of said cavities being coupled asymmetrically to saidwaveguide for a propagated signal of a frequency to which the cavity isresonant, and

means for tuning said cavities to resonance successively in the sameorder in which they are positioned around the waveguide whereby saidpropagated signal is conically scanned when radiated.

4. In an antenna feed for providing angular deflection of signals withinone frequency band while simultaneously passing undeected signals withinother frequency bands, the combination comprising:

an oversized waveguide having two symmetry planes,

a transmission line coupled to said waveguide, said line beingdimensioned to propagate said signals in a dominant mode such as the TEMmode or the TEM mode,

said waveguide being dimensioned so that it can propagate said signalsin the additional modes TVE-20, TEM and TMn,

a tunable cavity mounted on one side of said waveguide,

means for only coupling said cavity to said waveguide when a signalhaving a frequency within said one frequency band propagated throughsaid waveguide and for passing uncoupled and undeected said signalswithin said `other frequency bands,

said coupling means bein-g located asymmetrically with respect to thelongitudinal center axis of said one side of the waveguide whereby saiddominant mode of only said signals within said one frequency band issplit int-o said additional T1520, TEM and TMH modes and only saidsignals within said one frequency band are deflected.

5. An antenna feed for providing conical scanning, said feed comprisingan oversized waveguide having two symmetry planes,

a transmission line connected to said waveguide for propagating to itsignal in a dominant mode,

said waveguide being dimensioned so as to support the propagation ofsaid signal in higher-order modes,

a resonant cavity mounted on each side of said waveguide, each cavityhaving a coupling aperture extending into the waveguide,

each coupling aperture being located asymmetrically with respect to thelongitudinal center axis of the waveguide side in which it is located,each cavity being coupled t0 said waveguide when the cavity is resonantto a signal passing through said waveguide whereby said signal is passedthrough said waveguide with said dominant mode of propagation split intosaid higher-order modes, and

means for tuning the successive cavities positioned around saidwaveguide successively to resonance in a sine wave cycle so that eachcavity is resonated degrees out of phase with the preceding cavity.

6. In an antenna feed for providing angular deflection of signals withinone frequency band While simultaneously passing undeflected signalswithin other frequency bands, the combination comprising:

an oversized waveguide,

a transmission line coupled to said waveguide, said line beingdimensioned to propagate said signals within said one frequency band andsaid other frequency bands in the TElo mode,

said waveguide being dimensioned so that it can propl l agate saidsignals in the additional modes TEZO, TEn and TMm,

a tunable cavity mounted on one side of said waveguide,

means for only coupling said cavity to said waveguide when a signalhaving a frequency within said one frequency band is propagated throughsaid waveguide and for passing uncoupled and undeflected said signalswithin said other frequency bands,

said coupling means being located asymmetrically with respect to thelongitudinal center axis of said one side of the waveguide whereby saidTEU, mode of only said signals within said one frequency band is splitinto said additional T1320, TEU and TMll modes and only said signalswithin said one frequency `band are deflected.

7. An antenna feed for providing conical scanning, said feed comprisingan oversized waveguide,

a transmission line connected to said waveguide for propagating to itsignal in a dominant mode,

said waveguide being dimensioned so as to support the propagation ofsaid signal in higher-order modes,

a resonant cavity mounted on each side of said waveguide, each cavitybeing coupled asymmetrically to said waveguide when the cavity isresonant to a signal passing through said waveguide whereby said signalis passed through said waveguide with said dominant mode of propagationsplit into said higherorder modes, and

means for tuning the successive cavities positioned around saidwaveguide successively to resonance in a sine wave cycle so that eachcavity is resonated 90 degrees out of phase with the preceding cavity.

8. An antenna feed comprising an oversized waveguide having twoorthogonal symmetry planes,

a matching waveguide section,

a transmission line coupled to said oversized waveguide through saidmatching section, said line being dimensioned to propagate signals ineither the TEN mode or the TEM mode or in both of said modes,

said waveguide being dimensioned so that it can propagate signals in theadditional modes TE20, TEU and TMll,

means for introducing asymmetry into said waveguide whereby saidfirst-mentioned modes are split into said additional TE20, TEM and TMllmodes,

said last named means including a resonant cavity mounted on each sideof said waveguide, each of said cavities being coupled asymmetrically tosaid waveguide for a signal propagated in at least one of saidfirst-mentioned modes and of a frequency to which the cavity isresonant, and

means for tuning said cavities to resonance successively in the sameorder in which they are positioned around the waveguide whereby saidpropagated signal is conically scanned when radiated.

9. An antenna feed comprising an oversized waveguide having twoorthogonal symmetry planes,

a transmission line coupled to said waveguide, said line bein-gdimensioned to propagate signals in either the TEN, mode or the TEM modeor in both of said modes,

said waveguide being dimensioned so that it can propagate signals in theadditional modes TE20, TEM and TMH,

means for introducing asymmetry into said waveguide whereby saidfirst-mentioned modes are split into said additional TE20, TEU and TMHmodes,

said last means including a resonant cavity mounted on each side of saidwaveguide, each of said cavities being coupled asymmetrically to saidwaveguide for a propagated signal of a frequency to which the cavity isresonant,

12 each of said cavities having a wide side and being positioned withits wide side tilted at a certain angle with respect to a line drawnperpendicular to the longitudinal axis of said waveguide, the value ofsaid certain angle being such that each cavity has an equal amount ofcoupling to both the TEM, and the TEM metry planes,

a transmission line coupled to said waveguide, said line beingdimensioned to propagate signals in a certain mode,

said waveguide being dimensioned so that it can propagate signals in theadditional modes T1520, TEn and TMll, ,v

means for introducing asymmetry` into said waveguide whereby saidcertain mode is split into said additional TE20, TEU and TMm modes, saidlast named means including a resonant cavity mounted on each side ofsaid waveguide,

each of said cavities being coupled asymmetrically to said waveguide fora propagated signal kof a frequency to which the cavity is resonant, and

means for tuning said cavities to resonance successivelyv in the sameorder in which they are positioned around the waveguide whereby saidpropagated signal is conically scanned when radiated, said means fortuning said cavities including means for varying the rate of saidsuccessive tuning so that the conical scanning of said radiated signalspeeds upand slows down in accordance with the variation of said rate.

11. An antenna feed for providing conical scanning,

said feed comprising an oversized waveguide, a transmission lineconnected to said waveguide for A propagating to it signal in a dominantmode,

said waveguide being dimensioned so as to support the propagation ofsaid signal in modes which are multiples or harmonics of said dominantmode,

a resonant cavity mounted on each side of said waveguide,

each cavity being coupled asymmetrically to said waveguide when thecavity is resonant to a signal passing through said waveguide wherebysaid signal is passed through said waveguide with said dominant Inode ofpropagation split into additional modes of propagation,

means for tuning the successive cavities positioned around saidwaveguide successively to resonance in a sine wave cycle so that eachcavity is resonated 90 degrees out of phase with the preceding cavity,and

means for making said sine wave cycle occur at successively differentfrequencies. i

References Cited UNITED STATES PATENTS 2,433,368 12/1947 Johnson et al343-786 X 2,905,940 9/1959 Spencer et al. 343-768 X 3,021,524 2/1962Kompfner 343-787 X HERMAN KARL SAALBACH, Primary Examinar.

WM. H. PUNTER, Assistant Examiner.

U.S. Cl. X.R.

